A thermoelectric conversion member is a material that is capable of directly converting thermal energy to electricity or directly converting electric energy to thermal energy, that is, performing heating or cooling by applying electricity.
When a plurality of pairs of p/n thermoelectric conversion members, in which a p-type thermoelectric conversion member and an n-type thermoelectric conversion member are combined, are electrically connected in series, one thermoelectric conversion module is formed. When using the thermoelectric conversion module, waste heat that conventionally has not been used so much may be converted into electricity, whereby energy may be effectively utilized.
Examples of a representative thermoelectric conversion member used for the thermoelectric conversion module, which are representative of research conducted up until now, include a Bi2Te3 system, a PbTe system, an AgSbTe2—GeTe system, a SiGe system, a (Ti, Zr, Hf) NiSn system, Skutterudite and filled Skutterudite systems represented by CoSb3, a Zn4Sb3 system, an FeSi2 system, NaCo2O4-based oxides, Ca3Co4O9-based oxides, and the like.
However, among these, only the Bi2Te3 system is in practical use. In the thermoelectric conversion module using the Bi2Te3-based thermoelectric conversion member, a temperature range that may be used for power generation is limited to a range from in the vicinity of room temperature to a maximum 250° C. that Bi2Te3-based material may endure.
Therefore, from the viewpoint of effectively using various kinds of waste heat, there has been a demand for a thermoelectric conversion module which may be used in an intermediate temperature range of 300° C. to 600° C. In recent years, particularly, as a thermoelectric conversion member that may be used in this temperature range, a thermoelectric conversion member having a filled Skutterudite structure has attracted attention.
A filled Skutterudite compound is expressed by a chemical formula of RT4X12, and has a cubic structure of a space group Im-3 (No. 204). In the formula, R represents an alkaline-earth metal and an element of lanthanoid series or actinoid series, T represents a transition metal such as Fe, Ru, Os, Co, Pd, and Pt, and X represents pnictogen elements such as As, P, and Sb.
Particularly, a thermoelectric conversion member of the filled Skutterudite system in which X is Sb has been actively studied. The thermoelectric conversion member of the filled Skutterudite system exhibits high thermoelectric performance at the intermediate temperature range.
When preparing the thermoelectric conversion module using the thermoelectric conversion member, it is necessary to bond respective p-type and n-type thermoelectric conversion members and electrode members at a high-temperature portion and a low-temperature portion. The thermoelectric conversion module using the Bi2Te3-based thermoelectric conversion member is used at a temperature range of 250° C. or lower.
Accordingly, this bonding is carried out according to a relatively easy method using solder, a brazing filler material, or the like without excessively considering an effect of heat. On the other hand, when preparing a thermoelectric conversion module that may be used at an intermediate temperature region of 300° C. to 600° C., material selection of an electrode member that connects a p-type thermoelectric conversion member and an n-type thermoelectric conversion member and material selection of a bonding method are important problems.
It is necessary that bondability is good between the electrode member and each of the thermoelectric conversion members and that performance deterioration of the thermoelectric conversion member due to the electrode member does not occur. To realize these, in a usage temperature range up to 600° C., matching properties in a thermal expansion coefficient between the thermoelectric conversion member, the electrode member, and a material used for bonding thereof and stability of a bonding layer at a bonding interface are necessary.
When a difference in thermal expansion coefficient is large, a large thermal stress is generated, and thus there is a problem in that fracture of a bonded portion occurs. In addition, when element diffusion progresses at the bonding interface, deterioration of a thermoelectric performance and a decrease in performance of the electrode member occur in the electrode member and the thermoelectric conversion member.
If the thermoelectric conversion module can be prepared using the filled Skutterudite-based thermoelectric conversion member, it is possible to use an element having high conversion efficiency in a further higher temperature region compared to the thermoelectric conversion module in which Bi2Te3 is used in the related art. However, solder may not be used at a bonding portion between the thermoelectric conversion member and the electrode member at a high-temperature portion.
In addition, antimony (Sb) which is a constituent component of the filled Skutterudite-based thermoelectric conversion member, an electrode member of copper (Cu) which is used in the related art, and a brazing filler material or a paste material for bonding of the electrode member react with each other, and thus, when these are bonded to each other, a deterioration of the constituent material occurs over the passage of time. Accordingly, the lifespan of the thermoelectric conversion module comes to an end without exhibiting the original performance of the thermoelectric conversion member, and there is a problem of durability.
With regard to the above-described problem, there is a suggestion for a thermoelectric conversion module in which titanium or an alloy layer of a titanium alloy is provided between the thermoelectric conversion member and the electrode member at the high-temperature portion related to the thermoelectric conversion member having the Skutterudite structure.
More specifically, in the thermoelectric conversion module including an n-type thermoelectric element and a p-type thermoelectric element, a titanium layer or a titanium alloy layer having a thickness of 10 μm or more is formed in at least one of the n-type thermoelectric element and the p-type thermoelectric element.
There is disclosed that a compound having a Skutterudite type crystal structure is used as a material of the n-type element, and for example, the following compounds are exemplified.
(1) Compound Expressed by M1-AM′AXB 
Here, M represents any one of Co, Rh, and Ir, M′ is a dopant for realizing an n-type and represents any one of Pd, Pt, and PdPt, X represents any one of As, P, and Sb, and it is preferable that conditions of 0<A≦0.2 and 2.9≦B≦4.2 be satisfied.
Particularly, when B is set to 3, a compound having a simple composition ratio may be obtained. As a specific example, a Co—Sb-based compound, for example, Cu0.9 (PdPt)0.1Sb3 may be exemplified. Here, instead of Co0.9(PdPt)0.1Sb3, CoSb3 having the same structure may be used.
(2) Compound Expressed by M(X1-AX′A)3 
Here, M represents any one of Co, Rh, and Ir, X represents any one of As, P, and Sb, X′ represents any one of Te, Ni, and Pd, and it is preferable that a condition of 0<A≦0.1 be satisfied.
(3) Compound Expressed by M1-AM′A(X1-BX′B)C 
Here, M represents any one of Co, Rh, and Ir, M′ is a dopant to realize an n-type and represents any one of Pd, Pt, and PdPt, X represents any one of As, P, and Sb, X′ represents any one of Te, Ni, and Pd, and it is preferable that conditions of 0<A≦0.2, 0≦B≦0.1, and C=3 be satisfied.
With regard to the above-described thermoelectric conversion module, in a thermoelectric conversion module using n-type and p-type thermoelectric elements having excellent characteristics in a high-temperature region in the vicinity of 500° C., element diffusion at a bonding portion and the like may be prevented (for example, refer to Patent Document 1).